1. Field of the Invention
The present invention relates to an anti-skid control system for controlling a braking force applied to road wheels in braking operation of a vehicle to prevent the road wheels from being locked, and more particularly to an anti-skid control system adapted for a part time four-wheel drive vehicle for selectively providing a four-wheel drive mode and a two-wheel drive mode.
2. Description of the Prior Art
In order to prevent road wheels from being locked, there has been employed an anti-skid control system which controls the braking force by decreasing, increasing, or holding a hydraulic braking pressure supplied to each wheel brake cylinder operatively connected to each road wheel. According to the anti-skid control system, in general, detected is a rotational speed of each road wheel or wheel speed, and the hydraulic braking pressure in each wheel brake cylinder is controlled in response to each wheel speed, so as to obtain the maximum coefficient of friction.
As for a driving system of the vehicle, there is known a so called part time four-wheel drive system, or selective four-wheel drive system, wherein a two-wheel drive (2WD) mode for driving either axle of front road wheels or that of rear road wheels, and a four-wheel drive (4WD) mode for driving the axles of both front and rear road wheels are selectively provided. In this driving system, the axles of front and rear road wheels are directly connected with each other or a differential operation is limited in the four-wheel drive mode, so that the wheel speed of each road wheel tends to be synchronous with the wheel speeds of other road wheels, especially on a road of a low coefficient of friction. Therefore, it is difficult for such part time 4WD vehicle to employ the anti-skid control system generally employed in the two-wheel drive vehicle, such as a three channel control system which provides an independent control for the front road wheels and a low-select control for the rear road wheels.
In order to provide an appropriate anti-skid control in the four-wheel drive mode for the part time 4WD vehicle, in Japanese Patent Laid-open Publication No. 62-251264, for example, it has been proposed to change the braking force control into that of whole wheels simultaneous control in the four-wheel drive mode by means of a changeover switch. However, the changeover switch is expensive and likely to be damaged. Further, in the case where the braking force control is to be changed by means of the changeover switch, the braking force control is not necessarily appropriate in braking operation on a road of a high coefficient of friction. On the road of a high coefficient of friction, stopping distance may be shortened through the braking force control for the two-wheel drive mode even in the four-wheel drive mode.